Light effectively enters an eye substantially parallel to the eye's visual axis. In a normal eye, light is refracted inside the eye (i.e. at the anterior surface of the eye) so it will be incident on the retina in a manner that allows for the creation of a visual image. More specifically, it is known that the refraction of light at the anterior surface of an eye directs the incident light toward a common point on the eye's visual axis. Furthermore, based on the anatomical structure of an eye, this point is calculated to be generally located at a distance approximately 20 mm from the eye's anterior surface. Accordingly, the incident light will have an angle of refraction “φ” that depends directly on where the light is incident on the eye's anterior surface. Geometrically, when the light is incident on the anterior surface at a distance “s” from the eye's visual axis, the angle “φ” is equal to arc sin s/20. Importantly, light that is incident on the anterior surface of the cornea of an eye, at a distance “s” from the visual axis, will be refracted and directed along the surface of what is hereinafter referred to as a “refraction cone” or “refraction conical surface”. In each instance, this refractive cone is centered on the visual axis and has a decreasing taper in the posterior direction. The angle of taper for the refraction cone is equal to the refraction angle “φ” (i.e. arc sin s/20).
When incoming light is not refracted in the manner noted above, or is otherwise scattered in some way, visual distortions or hazy (cloudy) sensations can sometimes result in the image. It happens that these sensations may be caused by the very surgical attempts that were made to correct the underlying vision defect.
As recently disclosed in a U.S. patent application for an invention entitled “Computer Control for Bio-Mechanical Alteration of the Cornea,” which was filed on “Jan. 18, 2008,” and which is assigned to the same assignee as the present invention, the reshaping of a cornea to correct visual defects can be effectively accomplished by performing LIOB over all or portions of substantially cylindrical-shaped surfaces in the cornea. As implied above, if these surfaces do not account for the refraction angle “φ” (i.e. the surfaces do not eventually conform with the appropriate refraction cone) it is possible they will cause refractions that result in stray light being generated. As indicated above, this can cause unwanted visual sensations. Specifically, it is known that, under certain lighting conditions, this stray light will introduce a hazy or cloudy sensation into a patient's perceived visual image.
The elimination or effective minimization of stray light in the eye is, in large part, dependent on ensuring that abnormal refractive surfaces are not presented by the cornea. Stated differently, in order to avoid the introduction of stray light, when a surgical procedure is employed to produce so-called cylindrical cuts it is important that the surfaces (i.e. cuts) created during the procedure are effectively and properly oriented on a refraction cone relative to the visual axis; after the surgery. The issue then turns to how these surfaces are created, and how they are oriented during their creation.
In essence, any surface that is created by “cuts” into intrastromal corneal tissue, may cause stray light to be introduced. LIOB is a well known method for creating these cuts. Other known procedures, however, may also result in such “cuts.” Specifically, for one, when tissue is repeatedly impacted by a sequence of multiple laser pulses, the result can be a compromise of the tissue. A sufficient number of such impacts can then effectively result in the creation of a “cut.” This can happen, even though the multiple laser pulses (e.g. femto-second laser pulses) each has an energy level that is below the threshold for LIOB. Still, an unwanted consequence may be the introduction of stray light. With the above in mind, reference to LIOB in the disclosure below should be taken to include not only LIOB, as generally defined, but also other laser surgical procedures that create “cuts” to thereby create an intrastromal surface in the cornea
For purposes of ophthalmic surgery, in order to perform any LIOB, or LIOB-type, surgical procedure it is typically necessary to first stabilize the eye. In most instances, this eye stabilization is accomplished by engaging the anterior surface of the eye with a contact lens. To ensure an effective engagement, however, the contact lens needs to have a radius of curvature “Rc” that is greater than the anatomical radius of curvature “R” of the eye. As a consequence, when a contact lens is engaged with an eye, the eye and its cornea become deformed. LIOB is therefore performed on a deformed cornea. When the contact lens is then removed from the eye, the eye (and cornea) will naturally recover from the deformation. In this case, the predominant recovery forces will be provided by intraocular pressure (IOP) in the eye. To ensure that surfaces created by LIOB will become appropriately oriented relative to the visual axis of the eye after surgery, deformation of the cornea during the LIOB procedure requires compensation. And, this compensation needs correction by the refraction angle “φ” discussed above.
In light of the above it is an object of the present invention to provide for an effective alignment of LIOB created surfaces parallel to the visual axis, after a surgical procedure. Another object of the present invention is to provide for systems and methods that will compensate for anatomical deformations of a cornea during LIOB surgery by predicting the recovery trends of the cornea after the surgery has been performed. Still another object of the present invention is to provide for systems and methods for minimizing stray light caused by the LIOB of tissue on surfaces inside a cornea of a patient that are easy to employ and comparatively cost effective.